Understanding the Evolution of the Iron Carbonate (FeCO3) Layer and Its Effect on the Localised Corrosion of X65 Carbon Steel in CO2-Saturated Environments

Carbon steel pipelines are highly susceptible to localised corrosion attack in CO2-containing environments. Under certain conditions, the formation of corrosion products, specifically iron carbonate (FeCO3) layer, is possible. While most of the corrosion mitigation strategies involve the use of corrosion inhibitors, it is widely reported that FeCO3 layers can also have a direct influence on the corrosion behaviour of the metal, by forming dense, adherent layers that provide protection through active-site blocking and diffusion barriers. Both formation and quality of the resulting corrosion layer are strongly influenced by the local environment, operating conditions, as well as the properties of underlying pipeline material. Despite the fact that substantial progress has been made in understanding and characterising protective properties of FeCO3 layer, the mechanism behind localised corrosion initiation and propagation in the presence of such layers remains poorly understood. Furthermore, the effect of the FeCO3 layer on inhibitor efficiency adds another level of complexity.
This thesis investigates the initiation and propagation of the localised corrosion on the X65 carbon steel surface in the presence of the FeCO3 layer. Localised corrosion initiation and propagation mechanisms are assessed with respect to different stages of FeCO3 formation in CO2-saturated 3.5 wt.% NaCl electrolyte at 80 ºC, pH 6.8. A standard three-electrode test setup was used to obtain in-situ corrosion rate measurements; the corrosion layer was characterised using ex-situ SEM, EDX and XRD, and was found to be FeCO3. White-light surface interferometry was used to assess the extent of the localised corrosion beneath the FeCO3 layer. Severity of the localised corrosion attack was expressed as maximum pit depth, average pit depth and average diameter of the deepest pit. It was established that the localised corrosion initiates and propagates in conjunction with nucleation and formation of the FeCO3 crystals; in fact, local electrolyte entrapment directly under or in between the FeCO3 crystals is believed to be the main precursor to the localised corrosion initiation. In addition, the effect of pseudo-passivity of the FeCO3 layer on the propagation of the localised corrosion has been assessed. Cyclic potentiodynamic technique has been utilised to understand the stability of the pseudo-passive layer and the susceptibility of carbon steel to undergo pitting corrosion with the presence of such protective FeCO3 layer. It was found that the localised corrosion continues to propagate under the FeCO3 layer, even when pseudo-passivity has been achieved.
Finally, interactions between FeCO3 layer and imidazoline-based inhibitor have been assessed. 30 ppm and 100 ppm of inhibitor have been added into the bulk solution at different stages of pre-corrosion, corresponding to different levels of the FeCO3 layer coverage. It was found that the localised and general corrosion efficiency is dose-dependent as well as surface coverage area
dependent, with the most severe localised attack observed during the 100 ppm test and 12 hours of pre-corrosion, corresponding to partial surface coverage.